1. Field of the Invention
The present invention relates to a hydrostatic steering and all-wheel drive system for an articulated vehicle, especially for an articulated cross country or an off-road vehicle of the type containing a front part and a rear part, which are connected to each other by a hinge allowing the mutual turning movements of the parts essentially in the horizontal plane. Both of the vehicle parts are supported by one right-hand and one left-hand wheel, respectively, the wheels being furnished with hydraulic motors of their own. The hydraulic system includes two powered hydraulic pumps independently supplying pressurized hydraulic fluid into two separate hydraulic circuits. Both of the hydraulic circuits connect one front-wheel hydraulic motor and one rear-wheel hydraulic motor to one of the powered hydraulic pumps in order to achieve the movement of the vehicle in the forward direction and in the backward direction, respectively.
2. Prior Art
Vehicles of this kind have been described for instance in U.S. Pat. Nos. 3,151,694 and 3,865,208 including principally equal hydraulic systems to drive and steer a four-wheel driven articulated vehicle. The all-wheel driven articulated vehicles described in these patents include a usual steering system, which includes certain power means such as a hydraulic cylinder-piston arrangement by means of which the mutual angular position of the front and rear parts of the vehicle around the articulation point is changed in order to effect the steering of the vehicle.
All-wheel or four-wheel hydraulic driving systems are, however, encountered with various problems in use, which appear in different modes depending on the way in which the wheel driving hydraulic motors are connected to the pump or pumps for supplying hydraulic fluid into the motors. The hydraulic systems disclosed in these patents are principally intended to eliminate these drawbacks of the known systems, such as scuffing of the wheels on different sides of the vehicle when the vehicle moves along an arcuate path or the problems arising when one of the tractive wheels looses its contact with the ground or slips.
In order to solve these problems the system according to U.S. Pat. No. 3,151,694 uses two, essentially independent hydraulic systems, each connecting the hydraulic wheelmotors on one side of the vehicle in series to the common hydraulic pump supplying the hydraulic circuits with pressurized hydraulic fluid. The flow of the hydraulic fluid in both of the circuits can be controlled by a porportioning means, whereby a certain effected increase in flow rate in one of the circuits causes a corresponding decrease in flow in the other circuit. In a preferred embodiment of the system the control of the proportioning means is connected to the mutual angular position of the front and rear parts of the vehicle, whereby during the turning movement of the vehicle the motors driving the wheels at the inside of the turn receive a reduced portion of the fluid supplied by the pump and the motors at the outside of the turn receive a corresponding increased amount of the fluid, respectively. This kind of proportioning of the hydraulic fluid guarantees that the wheel-motors on opposite sides of the vehicle receive a hydraulic flow proportional to the travel path of the wheels.
It is, however, to be noted that the basic turning movement in the described vehicle is still achieved by a hydraulic cylinder-piston device, which is connected between the front and rear parts of the vehicle and that the object of the two-circuit hydraulic system, including the proportioning means is to assist the basic steering or turning means in their turning action, or to eliminate the negative effects caused by the turn in the mutual angular position of the vehicle parts to the driving conditions in the wheel-motors. There is no disclosure in the mentioned patent that in a four-wheeled driving embodiment of the vehicle the steering of the vehicle could be effected solely by using the proportioning means of the two independent hydraulic circuits. The only disclosure to this effect concerns the front-wheel driving embodiment of the system, which embodiment, however, is to be considered merely to relate to a vehicle intended for road conditions. By this two-wheel driving system the benefits of all-wheel drive are lost, which benefits are essential to a vehicle intended to move in terrain. Instead the two-wheel driving system alternative is beneficial when the vehicle moves a road, because the whole pumping capacity of the hydraulic pump can be used in each circuit to power the sole front-wheel motor, whereby the speed of the vehicle can be duplicated. In addition this mode of the driving system accomplishes a comprehensive way to steer the vehicle.
The system disclosed in FIG. 1 of the specification could be modified on the basis of the embodiment described in FIG. 2 thereof in such a manner that the circuits supplying hydraulic power to both wheels on one side would be controlled by the steering means of the vehicle in order to steer the vehicle. This kind of powering system would, however, be useless in practice, because the front and rear parts of the vehicle would tend to turn in the same direction, which would lead to very unstable and contradictory force direction conditions on the hinge connecting the two vehicle parts. The system disclosed in this patent has its own special problem arising from the connection of the wheel-motors on one side of the vehicle in series to the power supplying pump. There is thus no slackness between the front and rear wheels on one side of the vehicle which would eliminate the scuffing of the wheels during the initial phase of the turning movement, when the angle between the vehicle parts changes. In order to solve among other things the problems encountered in the system disclosed in U.S. Pat. No. 3,151,694, described above, the hydraulic system of U.S. Pat. No. 3,865,208 involves a preferred embodiment according to which each of the wheels powering hydraulic motors have their own hydraulic pump supplying hydraulic fluid to the motors. The hydraulic system disclosed in this patent contains also a proportioning system comparable to the system described in the earlier patent the purpose of which is also the same, i.e. to proportion the hydraulic fluid flows to the wheel motors on both sides of the vehicle in relation to the mutual angular position of the vehicle parts.
The steering of this vehicle is achieved also in conformity with the system disclosed in U.S. Pat. No. 3,151,694, whereby the hydraulic cylinder-piston arrangement mounted between the vehicle parts is activated when turning of the vehicle is desired. Nor does the later patents make any disclosure to the effect, that the steering action could be accomplished in any other manner than by using the cylinder-piston device. The only way to affect the steering movement of the vehicle using the wheels powering system disclosed in the patent is the abovementioned proportioning system which equally affects the wheel motors on the same side of the vehicle. Using solely this proportioning system for steering the vehicle would thus lead to the same indefinite situation on the hinge connecting the vehicle parts as mentioned in connection with the earlier specification. The only useful mode to steer the vehicle on the basis of U.S. Pat. No. 3,865,208 is thus obviously with the use of the hydraulic cylinder-piston device.
All the known steering systems for articulated four-wheel driven cross-country vehicles include a power device such as a hydraulic cylinder-piston arrangement mounted between the vehicle parts. The cylinder-piston arrangement has, however, encountered remarkable drawbacks in the vehicles intended to move in rough terrain, especially in vehicles provided with such a hinge between the vehicle parts, which allows the coaxial mutual twisting movements of the vehicle parts around the longitudinal axis of the vehicle. When this kind of vehicle moves in rough terrain it might come to a situation where the front part and the rear part of the vehicle, respectively are on the ground with opposite inclination. In this situation the vehicle parts are remarkably twisted in relation to each other, whereby the cylinder-piston arrangement should yield in order that the actual intended steering angle between the vehicle parts should not be changed. This kind of compensating arrangement is, however, difficult to achieve, whereby a steering system eliminating the necessity to use the cylinder-piston device for steering of the device would be desirable.